Diode receiver for radio frequency transponder

ABSTRACT

A passive radio frequency transponder (RF tag) having a diode rectifier receiver circuit outside the tag power rectification circuit, the tag power rectification circuit supplying power to the electronics of the RF tag. An additional innovative low current circuit protect the signal capacitor from overvoltage produced by the signal diode. An innovative circuit also clips the signal and sharpens it. An innovative low current circuit is used as a comparator to sharpen the signal pulses.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a division of application Ser. No. 10/440,492filed May 16, 2003, which claims the benefit of provisional U.S.application No. 60/430,553 filed Dec. 3, 2002, and claims the benefit ofprovisional U.S. application No. 60,385,528 filed Jun. 4, 2002. Saidapplication Ser. No. 10/440,492 is a continuation-in-part of copendingnonprovisional U.S. application Ser. No. 10/308,859 filed Dec. 3, 2002now abandoned. Said application No. 10,308,859 is a continuation-in-partof U.S. application Ser. No. 10/162,418 filed Jun. 4, 2002, nowabandoned, which is a continuation of application Ser. No. 09/426,235filed Oct. 25, 1999, now U.S. Pat. No. 6,400,274 issued Jun. 4, 2002,which is a continuation of U.S. application Ser. No. 09/321,986 filedMay 28, 1999. Said U.S. application Ser. No. 10/162,418 is also acontinuation-in-part of U.S. application Ser. No. 09/227,768 filed Jan.8, 1999, now U.S. Pat. No. 6,243,013 issued Jun. 5, 2001. Saidapplication Ser. No. 09/321,986 is a continuation-in-part of applicationSer. No. 08/733,684 filed Oct. 17, 1996 now U.S. Pat. No. 5,889,489issued Mar. 30, 1989 which in turn is a continuation-in-part ofapplication Ser. No. 08/521,898 filed Aug. 31, 1995, now U.S. Pat. No.5,606,323 issued Feb. 25, 1997. Said U.S. application Ser. No.10/162,418 is also a continuation-in-part of U.S. applications No.09/114,037 filed Jul. 10, 1998, now abandoned, No. 09/195,733 filed Jan.19, 1998, now abandoned, and No. 09/211,584 filed Dec. 14, 1998, whichis a continuation of application Ser. No. 08/626,820 filed Apr. 3, 1996,now U.S. Pat. No. 5,850,181 issued Dec. 15, 1998. Said U.S. applicationSer. No. 09/321,986 is also a continuation-in-part of application Ser.No. 09/263,057 filed Mar. 6, 1999, now abandoned, which claims thebenefit of U.S. Provisional application No. 60/077,094 filed Mar. 6,1998 and a continuation-in-part of application Ser. No. 09/266,973 filedMar. 12, 1999, now abandoned, which claims the benefit of provisionalapplication No. 60/077,872 filed Mar. 13, 1998. application Ser. No.09/922,598 filed Dec. 29, 1998, U.S. Provisional application No.60/070,347 01/02/98, U.S. Provisional application No. 60/385,528 filedJun. 4, 2002, U.S. Provisional application No. 60/430,553 filed Dec. 3,2002, and U.S. Pat. Nos. 6,400,274, 6,243,013, 6,028,564, 6,097,347,5,808,550 and 5,606,23 are each incorporated herein by reference in itsentirety. All of the above identified patents and patent applicationsare hereby incorporated herein by reference in their entirety includingincorporated material.

BACKGROUND OF THE INVENTION

The field of the invention is Radio Frequency (RF) transponders (RFTags) which receive RF electromagnetic radiation from a base station andsend information to the base station by modulating the load of an RFantenna.

RF Tags can be used in a multiplicity of ways for locating andidentifying accompanying objects, items, animals, and people, whetherthese objects, items, animals, and people are stationary or mobile, andtransmitting information about the state of the of the objects, items,and people. It has been known since the early 60's in U.S. Pat. No.3,098,971 by R. M. Richardson, that electronic components on atransponder could be powered by (RF) power sent by a “base station” at acarrier frequency and received by an antenna on the tag. The signalpicked up by the tag antenna induces an alternating current in theantenna which can be rectified by an RF diode and the rectified currentcan be used for a power supply for the electronic components. The tagantenna loading is changed by something that was to be measured forexample a microphone resistance in the cited patent. The oscillatingcurrent induced in the tag antenna from the incoming RF energy wouldthus be changed, and the change in the oscillating current led to achange in the RF power radiated from the tag antenna. This change in theradiated power from the tag antenna could be picked up by the basestation antenna and thus the microphone would in effect broadcast powerwithout itself having a self contained power supply. In the citedpatent, the antenna current also oscillates at a harmonic of the carrierfrequency because the diode current contains a doubled frequencycomponent, and this frequency can be picked up and sorted out from thecarrier frequency much more easily than if it were merely reflected.Since this type of tag carries no power supply of its own it is called a“passive” tag to distinguish it from an active tag containing a battery.The battery supplies energy to broadcast the information from the tagantenna. An active tag may also change the loading on the tag antennafor the purpose of transmitting information to the base station.

The “rebroadcast” of the incoming RF energy at the carrier frequency isconventionally called “back scattering”, even though the tag broadcaststhe energy in a pattern determined solely by the tag antenna and most ofthe energy may not be directed “back” to the transmitting antenna.

In the 70's, suggestions to use tags with logic and read/write memorieswere made. In this way, the tag could not only be used to measure somecharacteristic, for example the temperature of an animal in U.S. Pat.No. 4,075,632 to Baldwin et. al., but could also identify the animal.The antenna load was changed by use of a transistor. A transistor switchalso changed the loading of the transponder in U.S. Pat. No. 4,786,907by A. Koelle.

A combination diode rectifier circuit and balanced modulator formodulating the antenna current at twice the carrier frequency wasproposed by Gary T. Carroll in U.S. Pat. No. 4,724,427.

Prior art tags have used electronic logic and memory circuits andreceiver circuits and modulator circuits for receiving information fromthe base station and for sending information from the tag to the basestation.

The continuing march of semiconductor technology to smaller, faster, andless power hungry has allowed enormous increases of function andenormous drop of cost of such tags. Presently available research anddevelopment technology will also allow new function and differentproducts in communications technology. The use of the prior arttransistor switches to change the loading of the transponder antenna andto receive information, however, leads to increased cost in the use of atotally integrated system consisting of a single chip connected to anantenna. The transistor switch of the prior art must be fast enough andhave low capacitance to work well contained on a chip in a reasonabletime. Such transistors lead to increased costs in the chipmanufacturing, as the entire chip must be made with the same technologyand the entire chip does not need the speed of the one transistorelement. The range of the communication distance from the base stationto the tag is critical. This range is determined by the voltage built upby the antenna and rectifying circuits on the tag. Passive RF tags mustdo two things which are incompatible. First, there must be a steadysupply voltage extracted from the modulated RF field to power thedevices on the tag. Second, there must be a data signal recovered fromthe modulated RF field which has well defined zeros and ones for use bythe tag digital electronics. If the signal is taken off from the voltageon the main power supply capacitor of the tag, the voltage swing must below to provide good power for the electronics, and high to provide goodsignal.

The information receiving sections of prior art RF tags draw down themain power supply capacitor which supplies power to the tag when no RFpower is sent from the base station. This is wasteful of energy anduseless, since there is no information to be received when the RF poweris off.

Prior art tags have modulating circuits and receiver circuits whichreduce the voltage which can be produced by the rectifier circuits.Prior art tags have circuits which require relatively high current,which reduces the voltage built up by the antenna and rectifyingcircuits on the tag.

RELATED APPLICATIONS

Copending patent applications assigned to the assignee of the presentinvention and hereby incorporated by reference, are:

-   -   Ser. No. ______ Sep. 9, 1994 entitled RF Group Select Protocol,        by a et. al;    -   Ser. No. ______ Sep. 9, 1994 entitled Multiple Item RF ID        protocol, by Chan et. al;    -   Ser. No. ______ Aug. 31, 1995 entitled Diode Modulator for RF        Transponder by Friedman et al.;    -   application submitted Aug. 9, 1996 entitled RFID System with        Broadcast Capability by Cesar et al.; and application submitted        Jul. 29, 1996 entitled RFID transponder with Electronic        Circuitry Enabling and Disabling Capability, by Heinrich et al.

OBJECTS OF THE INVENTION

It is an object of the invention to produce an RF transponder comprisingcircuits which can be made at low cost. It is a further object of theinvention to produce an RF transponder which can be used at highfrequencies. It is a further object of the invention to produce an RFtransponder with maximum range. It is a further object of the inventionto produce an RF transponder with circuits which require very littlecurrent. It is a further object of the invention to produce anelectronic chip for an RF transponder which can be produced simply withstandard semiconductor manufacturing techniques. It is a further objectof the invention to produce a communication system for communicatingwith the RF transponder of the present invention. It is a further objectof the invention to produce a system for controlling the communicationsystem using the present invention. It is a further object of theinvention to produce a system for using and changing informationreceived from the transponder of the present invention.

SUMMARY OF THE INVENTION

The invention provides a diode receiver of a passive RF Transponderwhich is not part of the rectifier power supply circuit to measure thecommunication signal to the tag. The present invention has furtheradvantages that the same rectification means used in the receivercircuit can also be used as a modulator. The present invention includesinnovative protection means for protecting the receiver circuitry. Thepresent invention includes innovative means for comparing a receivedanalog signal with the moving average of the analog signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. A partial block diagram of the circuits of an RF tag.

FIG. 2. Voltage doubling power supply for tag shown with diode receiveroutside tag power circuit and with resistor as current drain.

FIG. 3. Circuit for a preferred embodiment of the invention

FIG. 4 a RF power sent to the tag vs time

FIG. 4 b. Voltage across power capacitor vs time.

FIG. 4 c. Raw signal voltage VSIG across signal capacitor vs time.

FIG. 4 d. A time expanded sketch of the signal voltage VSIG acrossresistor current drain

FIG. 4 e. A time expanded sketch of the signal voltage VSIG acrosstransistor current drain.

FIG. 4 f. A time expanded sketch of the signal voltage VSIG withoptional hysteresis circuit.

FIG. 5. A preferred embodiment of the invention.

FIG. 6. An alternative implementation of a moving average circuit.

DETAILED DESCRIPTION OF THE INVENTION

The invention is to use a diode arrangement separate from therectification section supplying power to the chip as sketched in FIG. 1.

FIG. 1. is a block diagram of a tag antenna 70, a tag rectificationpower supply 2, a tag receiving section 4, comprising an RF diode 40 anda tag signal capacitor 50, a tag signal capacitor drain section 6, and again section 80 for producing digital signals from the analog signalvoltage VSIG produced across signal capacitor 50 by RF diode 40.Optional VSIG averaging and compare circuit 7, protection circuit 8, andhysteresis circuit 9 are also shown. Additional tag electroniccomponents and memory elements are not shown.

The main power supply diodes 10 and 20 feed current to the main powersupply capacitor 30 in block 2 in the voltage doubling scheme shown inFIG. 2. While a simple voltage rectification and doubling scheme isshown in the diagram, other full or partial wave rectification schemesas known in the art, and voltage doubling or other-voltagemultiplication and addition schemes known in the art of power supplies,could be used as well. The raw power fed to capacitor 30 is conditionedby block 34 which has a voltage protection circuit and which supplies asteady and reliable chip power supply voltage VDD as output to run thetag electronics (The connections for powering the blocks are not shown).Several other voltage sources needed by the tag electronics mayoptionally be produced by the power supply 2. These are shown in FIG. 1as VPMR and VNMR, and will be explained later.

RF Diode 40 which is separate from the tag power rectification circuit 2feeds current to charge signal capacitor 50. The signal capacitor 50charges up rapidly when the RF field (which is amplitude modulated)changes from the zero to the one state for on-off key signal modulation.(Amplitude modulation schemes where the RF carrier does not drop to zeroare anticipated herein, but the examples given are for a 100% amplitudemodulation of the carrier signal. Other modulation schemes such as phaseand frequency modulation also anticipated.) The voltage VSIG whichappears across the signal capacitor 50 is used to produce a digitalsignal. When the base station turns off the RF field in order tomodulate the RF signal to send information to the tag, the charge storedin signal capacitor 50 is drained off by the signal capacitor currentdrain section 6, which in the embodiment of FIG. 2 comprises a resistor60. The signal capacitor 50 does not then further drain current from thecapacitor 30 during the time where the RF power is turned off, and thecapacitor 30 can power circuits such as a clock circuit for a longertime than if the receiver circuit were draining current. The RC timeconstant for draining signal capacitor 50 must be short compared to thepulse length of the pulse modulation of the RF. If the RC time constantis long, the waveform recovered from the field modulation will be badlydistorted, making it difficult to generate the correct recovered bitpattern on the chip. If the resistor 60 is very large, there is not muchcurrent draw to load down the antenna 70, but the time constant becomeslong for a reasonable size signal capacitor 50. If signal capacitor 50is too small, it does not act as an RF short, and RF can leak into thegain circuit 80 and perturb the tag electronics with unpredictableresults. Furthermore, with a small signal capacitor 50, the circuit ismore susceptible to noise and performance values vary strongly withparasitic capacitance changes. VSIG is sent to a gain circuit 80 vialine 62 where it is turned into the clipped ones and zeros needed forthe digital electronics circuits. Such methods of turning a modulatedanalog signal into a digital signal are well known to one skilled in theart. The resimicrosecond time constant and a reasonable size signalcapacitor 50 is also very expensive to build on a chip, because largeresistors take up a lot of chip area. In addition, the current drainthrough a resistor will have a strong dependence on VSIG.

A more preferred embodiment of the invention is a novel circuit sketchedin FIG. 3. In place of the resistor 60 used to drain down the signalcapacitor 50 when the RF is shut off, the FET 120 is used. In contrastto the circuit of FIG. 2, a constant current is drawn through FET 120independent of the voltage VSIG. The magnitude of the current throughthe n-FET 120 is determined by a voltage VNMR on line 132. VNMR isderived from a constant current source (not shown) in the power supply2. Because the current is drawn from the signal capacitor 50 both whenthe RF field is on and when it is off, the amount of current drained byFET120 must be chosen carefully. The current must be large enough for aquick discharge of signal capacitor 50 when the field turns off, yetsmall enough that the recharging of the power capacitor 30 is minimallyaffected when the field turns on. VNMR is produced by a well knowntechnique of mirroring the current in a well known low current referencegenerator circuit. VNMR is very stable with respect to the circuitground, and is relatively independent of the (possibly) fluctuatingvoltage VDD produced by the power rectification circuit of the tag. Thecurrent through n-FET 120 is thus determined by the current throughanother n-FET, the determination being made principally by thedimensional relationships of the two FETs.

The modulated RF power sent to the tag is sketched in FIG. 4 a, and thevoltage VDD across the capacitor 30 is sketched in FIG. 4 b. In thesample data pattern shown, data is Manchester encoded, meaning that theorder of two half bits of different polarity determines the bit valuebeing sent. Here, a half-bit 1 followed by a half-bit 0 denotes a 1while a half-bit 0 followed by a half-bit 1 denotes a 0. The voltageacross resistor 60 is the raw signal voltage VSIG which is sketched inFIG. 4 c. FIG. 4 d shows a time expanded sketch of the signal voltageVSIG across resistor 60 of FIG. 2, while FIG. 4 e shows a time expandedsketch of the signal voltage VSIG when the n mirror FET 120 of FIG. 3 isused FIG. 4 f is a time expanded sketch of the signal voltage VSIG whenan optional hysteresis circuit 9, discussed later, is used with the nmirror controlled FET of FIG. 3.

The voltage VSIG is shown in FIG. 4 d as a function of time for a singlepulse with expanded time scale for the circuit shown in FIG. 2. Notethat the falling edge of the pulse is an exponential with a timeconstant given by RC, where R is the resistance of resistor 60 and C isthe capacitance of signal capacitor 50. In contrast, the voltage fallslinearly with the circuit of FIG. 3, as sketched in curve 4 e. The timetaken for VSIG to reach zero is determined by VNMR. The transistorcurrent drain 120 takes up much less space on the chip than a resistorwhich would give an adequate RC time constant.

An additional preferred embodiment of the invention is shown in FIG. 5.The voltage VSIG on signal capacitor 50 is averaged in an innovative wayin block 7 over a time which may be comparable or may be short comparedto an RF modulation frequency half cycle. The instantaneous voltage VSIGis then compared in block 7 to this moving average and when theinstantaneous voltage VSIG drops to some threshold compared to themoving average, the gain circuit 80 drains the signal capacitor 50 muchfaster than the transistor current drain current drain 120 can to give agood falling edge to VSIG. The gain circuit 80 sends a spike voltage totransistor 590 in block 9 of FIG. 5. The novel method of taking theaverage sketched in FIG. 5 is preferred to a standard averaging circuitusing a capacitor and a resistor which is well known in the art for thesame reasons that the circuit of FIG. 3 is preferrepacitor circuit showntakes less chip area and is more controllable and less variable than thestandard capacitor and resistor arrangement for averaging a voltage. Thep-mirror set up shown in FIG. 5 uses p-FET 510 and a current defined byVNMR thorough the n-FET 520 to define a voltage VPMR_LOCAL, which isdefined with respect to VSIG. The p mirror circuit shown is well knownin the art as a way of defining a voltage with respect to anothervoltage which is not at ground potential. The averaging capacitor 525charges and discharges relatively slowly, so that VPMR_LOCAL is relatedto a moving average of VSIG. VPMR_LOCAL is used to compare the presentvalue of VSIG with its moving average in a voltage comparator pull uppull down circuit, where p-FET 540 is controlled by VPMR_LOCAL to passtwice the current as transistor 510 in saturation mode, and n-FET 550 iscontrolled by VNMR to pass the same current as transistor 510 insaturation mode. When VSIG is steady, the voltage VPUPD is high, sincetransistor 540 can supply twice the current that transistor 550 needs.However, when VSIG drops a by a percentage, preferrably 25% and morepreferably 10%, voltage VSIG minus voltage VPMR_LOCAL drops by a verymuch higher percentage, while VNMR is unchanged with respect to ground,and transistor 540 can not supply the current needed to transistor 550,so VPUPD drops rapidly to a very low value. The voltage VPUPD is thenused by the gain stage 80 as the signal voltage.

An innovative optional protection circuit 8 is also shown in FIG. 5 toprotect the signal path electronics 50, 7, 80, and 6. Since the diode 40rectifying the incoming RF from the antenna 70 is not part of the chippower supply, the voltage VSIG may build up to large values if the tagis close to the base station. The signal capacitor 50 would then be atrisk unless some means of protecting it such as protection circuit 8were implemented. Protection circuit 8 is innovative in that it requiresless current and much less space on the chip than the conventionalprotection circuits used for the main power section included in block 2.The voltages developed elsewhere on the chip can be used in a novelfashion to cut down the number of devices needed by the protectioncircuit. Block 2 provides a voltage VDD which powers the chip circuits,and VDD is regulated and limited Block 2 also contains a p mirrorcircuit which provides a voltage VPMR related to VDD and a n-mirrorcircuit which provides a voltage VNMR related to ground. VPMR controlsthe gate on the p mirror p-FET 560. If VSIG is less than or equal toVDD, FET 560 demands ¼ the current of 570; thus, the voltage on “shunt”,the gate voltage of the high current FET 580, is low and FET 580 is off.If VSIG>VDD, by an amount on the order of tenths of a volt, (VSIG -VPMR)will be large enough to cause FET 560 to source more current than FET570 can sink. Thus, the gate of the high current FET 580 will be pulledup, turning on FET 580 and acting to pull down VSIG, which will providethe desired protection. This protection circuit draws less than 50 nAwhen there is no overvoltage condition. The specific ratio of currentdemanded by FET 560 versus FET 570 when VSIG is less than or equal toVDD is not critical, but is preferably substantially less than 1. If theratio is near or above 1, the shunt may turn on when VSIG is less thanVDD, draining power from the field into the signal path unnecessarily.

An optional feedback circuit is shown as block 9 in FIG. 5 to provide ahysteresis in the signal measuring circuit. When the gain circuit 80detects a falling edge on VSIG, circuit 80 provides a voltage pulse to ahigh current transistor 590 to short signal capacitor 50 to ground. Thetransistor 590 preferably can carry VSIG in this case is shown in FIG. 4f.

The p-channel transistors 510, 540, and 560 of FIGS. 5 and 610 and 640of FIG. 6 may optionally have their n-wells connected to VSIG or VDD.There is less noise when the n-wells are connected to VDD. The n-channeltransistors of FIG. 5 have their bulk contact connected to ground.

FIG. 6 shows an alternative implementation of a moving average circuitwhere the roles of the p-channel and n-channel devices have beenreversed. Here, VPMR is used to generate VNMR_LOCAL for the movingaverage generation.

While the particular circuits shown in FIGS. 5 and 6 are preferredbecause they are particularly suited to low current operation and usethe least number of devices and chip area possible, it is anticipated bythe inventors that a number of equivalent circuits are possible whichperform the same functions as the circuits shown. In particular,circuits where the roles of the p-channel devices and the n-channeldevices are reversed are anticipated.

1. A passive radio frequency (RF) transponder (tag), comprising: a tagantenna for receiving RF power and modulated RF information signals sentto the tag by a base stations; a tag voltage rectification power circuitattached to the tag antenna, the tag voltage rectification power circuitfor receiving RF power from the antenna and for providing power to tagelectronics, the tag electronics receiving power only from the tagvoltage rectification power circuit; and a tag voltage rectificationreceiver circuit separate from the tag voltage rectification powercircuit, the voltage rectification receiver circuit for receivingmodulated RF signals from the antenna and for providing demodulatedinformation signals to the tag electronics.
 2. The RF tag of claim 1,wherein the tag voltage rectification receiver circuit comprises; an RFdiode, a first terminal of the RF diode attached to attached to a firstterminal of the antenna; a signal capacitor attached between a secondterminal of the RF diode and a second terminal of the antenna, and; aresistor connected in parallel to the signal capacitor.
 3. A passiveradio frequency (RF) transponder tag, comprising: a tag antenna forreceiving RF power and modulated RF information signals sent to the tagby a base station; a tag voltage rectification power circuit attached tothe tag antenna, the tag voltage rectification power circuit forreceiving RF power from the antenna and for providing power to tagelectronics, the tag electronics receiving power only from the tagvoltage rectification power circuit; and a tag voltage rectificationreceiver circuit comprising a rectifier separate from the tag voltagerectification power circuit, the voltage rectification receiver circuitfor receiving modulated RF signals from the antenna and for providingdemodulated information signals to the tag electronics.
 4. The RF tag ofclaim 3, wherein the tag voltage rectification receiver circuitcomprises; an RF diode, a first terminal of the RF diode attached toattached to a first terminal of the antenna; a signal capacitor attachedbetween a second terminal of the RF diode and a second terminal of theantenna, and; a resistor connected in parallel to the signal capacitor.5. The RF tag of claim 3, wherein the rectifier comprises; an RF diode,a first terminal of the RF diode attached to attached to a firstterminal of the antenna; a signal capacitor attached between a secondterminal of the RF diode and a second terminal of the antenna, and atransistor current drain connected in parallel to the signal capacitor.6. The RF tag of claim 5, wherein the transistor current drain limitsthe current drained from the signal capacitor to a defined current. 7.The RF tag of claim 5, wherein the transistor current drain is an nchannel transistor connected as an n mirror transistor.
 8. The RF tag ofclaim 5, wherein the signal capacitor is protected against overvoltageby a low current protection circuit connected in parallel with thesignal capacitor.
 9. The RF tag of claim 8, wherein the low currentprotection circuit draws less that fifty nanoamperes (50 nA) when thereis not overvoltage on the signal capacitor.
 10. the RF tag of claim 8,where in the low current protection circuit comprises; a high currenttransistor connected in parallel with signal capacitor, a first andsecond low current transistor connected in series, each transistorhaving a different saturation current, the first and a second lowcurrent transistor connected in series being connected in parallel withthe signal capacitor, wherein a voltage at the connection between thefirst and a second low current transistor controls the high currenttransistor to carry high current to protect the signal capacitor duringovervoltage conditions, and to carry very little current in a conditionof no overvoltage.
 11. The RF tag of claim 5, wherein an instantaneousvoltage on the signal capacitor is compared to a moving average of theinstantaneous voltage on the signal capacitor.
 12. The RF tag of claim11, wherein the signal capacitor is shorted out when the instantaneousvoltage on the signal capacitor is significantly less than the movingaverage of the instantaneous voltage of the signal capacitor.
 13. The RFtag of claim 11, further comprising a moving average comparison circuitcomprising; a series connected first and a second transistor connectedin parallel with the signal capacitor, the first transistor with itsdrain connected to the gate and drain of the second transistor, the gateof the first transistor connected to a mirroring reference voltage, thesource of the first transistor connected to the second terminal of theantenna, and the source of the second transistor connected to the secondterminal of the RF diode; an averaging capacitor connected between thedrain of the first transistor and the second terminal of the antenna;and a series connected third and a forth transistor connected inparallel with the signal capacitor, the third and the forth transistorhaving different saturation currents, the gate of third transistorconnected to the gate of the first transistor, the drain of the thirdtransistor connected to the drain of the forth transistor, the source ofthe third transistor connected to the second terminal of the antenna,the gate of the fourth transistor connected to the gate of the secondtransistor, and source of the fourth transistor connected to the secondterminal of the RF diode.
 14. The RF tag of claim 11, further comprisinga moving average comparison circuit comprising; a series connected firstand a second transistor connected in parallel with the signal capacitor,the drain of the first transistor connected to the gate and drain of thesecond transistor, the gate of the first transistor connected to amirroring reference voltage, the source of the first transistorconnected to the second terminal of the RF diode, and the source of thesecond transistor connected to the second terminal of the antenna; anaveraging capacitor connected between the drains of the first 2transistors and the second terminal of the RF diode; a series connectedthird and a fourth transistor connected in parallel with the signalcapacitor, the third and fourth transistor having different saturationcurrents, gate of the third transistor connected to the gate of thefirst transistor, the drain of the third transistor connected to thedrain of the fourth transistor, the source of the third transistorconnected to the second terminal of the RF diode, the gate of the fourthtransistor connected to the gate of the second transistor, and thesource of the fourth transistor connected to the second terminal of theantenna.
 15. The RF tag of claim 5, wherein the signal capacitor isshorted out when the tag electronics detects a falling edge on thesignal of the signal capacitor.
 16. The RF tag of claim 15, furthercomprising a signal capacitor shorting circuit comprising; a highcurrent transistor connected in parallel with the signal capacitor, andan edge detection circuit which generates a spike on falling edgesdetected on the signal at the second terminal of the RF diode, the spikeoutput of the edge detection circuit connected to the gage of the highcurrent transistor.
 17. A passive radio frequency (RF) transponder (tag)comprising: a tag antenna for receiving RF power and modulated RFinformation signals sent to the tag by a base station; a tag voltagerectification power circuit attached to the tag antenna, the tag voltagerectification power circuit for receiving RF power from the antenna andfor providing power to tag electronics, the tag electronics receivingpower only from the tag voltage rectification power circuit; and arectifier separate from the tag voltage rectification power circuit forreceiving modulated RF signals from the antenna and for providingdemodulated information signals to the tag electronics.